This is neuroplasticity, my smart, dear Sammy :)

All the experiences we go through, either conscious or unconscious, are external stimuli that influence our brain cells to a given extent. Our brain pathways change its connections upon experiences that are lived more or less repeatedly. In other words, with practice. Your reduced focus must be more linked to mental exhaustion than anything else, I believe! I suppose it is time you began to going to bed a little earlier Sammy :*

Take care of yourself my dear.